In the prior art, devices are known which employ thin layers of mesomorphic materials subjected to a thermo-optical effect to reproduce images transmitted at a distance. In a general manner, these devices comprise a cell in which the material, interposed between two transparent sheets, is preheated to a temperature which is lower by several degrees Centigrade than the transition temperature from the mesomorphic phase utilised (the smectic or cholesteric phase) either to a further mesomorphic phase or to an isotropic liquid phase. The mode of procedure is such that the layer is uniformly oriented, either due to previous processing of the face of the support sheets contacting the layer, or due to application of an alternating field; thus, the layer is perfectly and uniformly transparent.
A beam of light, generally selected within the near-infrared zone and intensity modulated by a signal transmitting the image to be recorded, sweeps the cell. When the energy supplied at one point of the cell by the beam has been adequate to locally heat the liquid crystal to beyond the transition point at which the latter is converted to an isotropic liquid, there is locally formed, during the rapid return to the initial mesomorphic phase, a disordered texture (known as a focal-conic structure) which is optically scattering; on the contrary, when the luminous energy supplied by the modulated beam has been inadequate, the corresponding point retains the initial, uniform orientation and remains transparent. On projecting the image of the cell on to a screen by an appropriate optical device, the non-scattering spots of the cell appear as brilliant spots and the diffusing spots as dark spots.
The focal-conic structure thus set up within the material in the mesomorphic phase is stable. The devices constituted in this manner are storage devices able to preserve the recorded image for a duration varying between several hours and several days. In order to erase the recorded image, it is possible either to scan the cell with the luminous beam at full intensity and to then slowly recool the layer, or to submit the layer to the action of an alternating field, with or without sweeping by the beam.
The erasive time is always relatively slow (0.1 s for the fastest device). Consequently, it is impossible to employ such systems to reproduce animated images, for example televised images, since it is known that, in order to avoid scintillation, the projection rate must be at least equal to some 20 images per second.